image-builder: retry docker build once on failure

The -current full build failed on 2026-09-23 after all ten steps had
passed, at layer export, losing a race on the containerd snapshotter's
content lock ("failed to open writer: ref ... locked for 74ms ...
unavailable"). Disk was not the issue this time (73G free). The build
cache pre-prune that works around this only runs under --force, so the
normal gated nightly build had no guard.

Add docker_build() to lib.sh: docker build, retried once. By the time
the export fails every step is cached, so the retry is little more than
a re-export. Used by build-full-image.sh and build-sbo-testbuild.sh,
the two scripts that export ~20-33G images; bootstrap.sh's image is
small and has never hit this.

Self-check covers success, one failure, and two failures with a stub.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/image-builder/lib.sh b/image-builder/lib.sh
index 8a22828..9c519ac 100644
--- a/image-builder/lib.sh
+++ b/image-builder/lib.sh
@@ -91,3 +91,18 @@ require_mount() {
     local dir="${base}/slackware64-${version}"
     [[ -d "$dir" ]] || _err "NAS tree not mounted: ${dir}"
 }
+
+# docker_build ARGS...
+# `docker build`, retried once on failure. The export of a large image can lose
+# a race on the containerd snapshotter's content lock ("failed to open writer:
+# ref ... locked for 74ms ... unavailable"); the -current full build did on
+# 2026-09-23 after all ten steps had passed. The steps are cached by then, so a
+# retry is little more than a re-export.
+# ponytail: blind single retry; a real step failure costs one extra attempt
+# (a full rebuild under --force/--no-cache). Match on the error text if that
+# ever matters.
+docker_build() {
+    docker build "$@" && return 0
+    _warn "  docker build failed; retrying once..."
+    docker build "$@"
+}